Group B Streptococcus (GBS) is a major bacterial pathogen for human neonates. The polysaccharide capsule of GBS appears to be an important factor in virulence. Capsular type III is the most common serotype in invasive neonatal infections, and is particularly associated with meningitis. Previous work has suggested that the unique structural features of the type III capsule, especially its terminal side chain sialic acid residues, account for the pathogenic potential of type III GBS. The goal of this proposal is to study directly the role of the type III GBS capsular polysaccharide in pathogenesis of GBS infections through biochemical and immunologic analysis of genetically manipulated type III GBS organisms and their gene products. Genes involved in capsular biosynthesis will be cloned in E. coli and the cloned gene products characterized with regard to their role in capsular biosynthesis. Transposon mutagenesis will be used to produce phenotypically altered capsule mutants in the background of a virulent type III GBS strain. Mutant capsular antigens will be characterized structurally and immunologically, and the effects of particular phenotypic alterations will be assessed in terms of resistance of mutant strains to opsonophagocytosis in vitro, and virulence in animals.